Phosphorus combustion furnace



Dec. 5, 1950 C. J- M FARLIN PHOSPHORUS COMBUSTION FURNACE Filed June 1,1946 MkLInZQU Char/es J McFar/m INVENTOR BY MAJ E M ATTORNEY fiatenteciDec. l fi PHOSPHORUS CUMBUSTION FURNACE Charles J. McFarlin, nearShefiielid, Ala assignor to Tennessee Valley Authority, a corporation ofthe United States of America Application June 1, 1946, SerialNo..673,885

(Granted under the act of March 3, 1883, as

amended April 30, 1928; 3'70 0. 'G. 757) 2 Claims.

The invention herein described may be manufactured and used by or forthe Government for governmental purposes without the payment to me ofany royalty thereon.

This invention relates to the art of furnace construction, particularlyadapted to the recovcry of valuable oxidation products, such as theproduction of phosphorus pentoxide and the recovery of phosphoric acidin the combustion of elemental phosphorus.

The principal object of this invention is to provide a furnace with acombustion chamber in which the combustion products may be cooledrapidly without dilution. Another object of this invention is to providea furnace in which material containing an appreciable portion ofslagforming impurities may be oxidized and such impurities may bedeposited from the oxidation products and readily removed from thefurnace. A further object of this invention is to provide a furnacewhich may be economically constructed and maintained and in whichvaluable material may be recovered by the combustion of substanceshaving a high calorific value. Other objects of this invention includethe provision for an improved method for the operation of the furnacedescribed herein in the production of phosphoric acid.

The usual method of producing phosphoric acid from phosphorus orphosphorus-bearing gases is to burn the phosphorus with air in acombustion chamber, thus oxidizing the phosphorus to P205, and thenhydrate the P205 to orthophosphoric acid.

The common method for hydrating the P205 is to pass the combustion gasesinto direct contact with a spray of water without prior cooling of thegases. The acid is then collected from the gases by an electrostaticprecipitator or other suitable mist-collecting device.

This method of cooling and hydration has several disadvantages:

l. The large volume of steam evaporated into the gases in cooling by thedirect introduction of liquid water results in a substantial increase intotal gas volume and decrease in acid concentration. This has the effectof requiring a larger precipitator and causing a greater stack loss fora given final acid concentration in the stack gases wasted to theatmosphere.

2. The excessive partial pressure of water vapor in the gases encouragesthe formation of weak or dilute acid. In order to obtain acid ofcommercially desirable strength, it is necessary to operate the hydratorand precipitator at a relatively high temperature,which results in highrates of cor rosion in the hydrator, precipitator, exhaust fan, conduitsand stack. l

3.- Combustion of phosphorus or smelting-furnace gases in a combustionchamber of ordinary firebrick or refractory construction results inextremely high temperatures attended by rapid deterioration ofbrickwork. If an attempt is made to overcome this difficulty by sprayingliquid water or steam into the combustion space, large amounts of strongacid are formed whose corrosive effect on the refractory walls more thanonsets the reduction in temperature. Where excess air is used forcooling the combustion cham-' her, the gases are diluted with added badeffect on precipitator efiiciency and capacity.

The above disadvantages may be avoided by cooling the combustionproducts before they ente the hydrator, and it has been proposed that atleast a portion of this cooling be obtained by transier of heat from thecombustion products to COOhIlg water flowing through tubes embedded inthe walls of the combustion chamber. Operations involving such aproposal are unsatisfactory since the combustion chamber walls cannot becooled uniformly by this method. The heat generated in the combustionchamber is so intense that means for conducting the heat awayuniformlyand keeping the wall area at a fairly uniform temperature at all pointsmust be provided; otherwise, hot spots develop on the wall surface withconsequent injury to the refractory and damage to the cooling tubes byunequal thermal expansion.

One well-Known method for the cooling of hot surfaces is to maintain afilm of water flowing over the surface. However, the use of walls cooledwith a water him in apparatus of this type is attended with majortechnical problems which have not been solved satisfactorily heretolore.

In the first place, the heat generated in the combustion chamber is sointense that ordinary refractory material does not conduct heat fromthecombustion gases to the cooling water at a high enough rate. As aresult, the gases are not cooled to the desired degree and the wallsdeteriorate because of the high temperatures.

Moreover, those particular construction ma-' terials which have thecharacteristic of high heat transfer rate also have othercharacteristics which make them unsuitable. Graphite, for example, has asufficiently high heat transfer coefiicient but it oxidizes appreciablyat tempera tures as low as 840 F. Since the temper'ature'of thecombustion gases even with adequate cooling ranges from 1200 to 2000 F.,the latter characteristic would appear to preclude the use of graphite.Also, graphite has a relatively porous structure and would be expectedto leak gases out or cooling water in. For these reasons, no one, as faras we know, has ever seriously considered the possibility of building aphosphorus combustion chamber of graphite or similar materials. It hasbeen generally considered that the directspray and cooling-tube methodswere the only ones which could be used.

The apparatus of the present invention is directed to a furnaceconstruction, wherein the combustion products are cooled rapidly byindirect means for the subsequent removal of valuable products andcomprises the combination of a refractory hearth, a graphitewall, meansfor cooling the exterior of said graphite wall and maintaining the innersurface thereof below a temperature at which it will oxidize when incontact with the oxidizing atmosphere within said chamber, and arefractory enclosure opposite said hearth.

the accompanying drawings which form a Part of the specification andwherein reference symbols refer to like parts wherever they occur,

Fig, l is a diagrammatic, vertical, sectional view of one form ofapparatus for the embodimerit of the present invention showing furnaceconstruction wherein the combustion chamber is provided with a graphitewall between the refractory hearth and roof Fig. 2 is a diagrammatic,vertical, sectional view of another embodiment of the present inventionparticularly adapted to operations involving the combustion of elementalphosphorus which does not contain a substantial amount of slag-formingimpurities, wherein the hearth is constructed of graphite and cooled bya body of cooling liquid.

Fig, 3 is a diagrammatic, vertical, sectional view of still anotherembodiment of the present invention particularly adapted to theoxidation,

Ofelemental phosphorus carrying an appreciable amount of slag-formingimpurities, wherein the hearth is constructed of suitable refractoryother than graphite and from which the slag deposited thereon may beremovedv as required.

In Fig, 1, the combustion chamber l. consists of a refractory hearth .3,a vertical, cylindrical wall 5 constructed of graphite shapes andcontaining no joints perpendicular to the line of heat flow from theinterior to the exterior thereof, and refractory dome l, the exterior ofwhich is protected by an outer steel shell 9. Elemental phosphorus orphosphate reduction furnace gas containing elemental phosphorus isadmitted through phosphorus burner ll, through dome 1, into combustionchamber l, together with controlled amounts of air admitted throughvalved air inlets l3, and through dome shell 9 The products ofcombustion of the elemental phosphorus carrying phosphorus pentoxide arepartially cooled in combustion chamber l by heat transfer throughgraphite wall 5, which is exter nally cooled by the distribution ofwater (from water supply not shown) at the top of the wall throughdistributor E5 to provide a flowing water film 'IT over the surface ofsaid wall 5. The combustion products so partially cooled are drawn fromcombustion chamber I through outlet l9.

In Fig. 2, hearth 2i, as well .as wall 5, is also constructed ofgraphite shapes with no joints perpendicular to the line of heat fiowthrough the hearth. This graphite hearth 2! rests in a metal housing 23,the rim of which does not extend upwardly more than the thickness of thehearth 2 This metal housing 23 in turn is substantially surrounded bybody of cooling water 25, the depth of which is less than the height ofthe housing rim. A flashing ring 21 is provided at the bottom ofgraphite wall 5 to deflect the falling film of water and prevent it fromentering the metal housing 23.

In Fig. 3, refractory hearth 29 constructed of suitable refractory otherthan graphite serves to collect deposits of slag-forming materials whichmay be present in some supplies of crude elemental phosphorus.Ordinarily, the amount of such material may not be excessive, but assuch a deposit accumulates on hearth 25] it may be withdrawn therefromthrough tap 3|.

It may be seen that certain problems encountered with combustion ofhigh-calorific material and the recovery of valuable products can besolved by the furnace construction described herein. The combustionchamber has vertical, cylindrical walls of graphite shapes containing nojoints perpendicular to the line of heat flow and may have a hearth oflike construction. The walls are unjacketed and are cooled by a film ofwater flowing over the outside surface. When a graphite hearth isemployed, it is rested on a steel housing, the rim of which does notextend upwardly more than the thickness of the hearth; this housing isin turn supported in a body of cooling water, the depth of which is lessthan the height of the housing rim. A flashing ring is also provided atthe bottom of the wall to deflect the falling film of water and preventit from entering the steel housing. A refractory dome which forms theupper enclosure for the chamber is protected by an outer steel shell. Aphosphorus burner head is mounted at the top of the dome and thecombustion gases leave the chamber through an outlet near the hearth andare thereafter further processed.

As an alternate construction, for use when the phosphorus being burnedhas a relatively high ash content, the lower walls and hearth may bemade of a steel shell lined with firebrick or other refractory material.The upper wall section is bare, unjacketed graphite as in the firstdesign. The purpose of therefractory lined lower section is to provide azone which can be subjected occasionally to an increased temperature byconcentration of combustion in the .hearth zone, in order to meltaccumulated ash and tap it out.

The use of graphite is the most distinctive feature of the design.Because of its unique property of high thermal conductivity and theparticular construction used, the graphite conducts away to the waterfilm the greater. part of the heat of combustion and is itself at thesame time relatively cool even on the inside sur face. Being alwayscool, the graphite walls are not damaged by oxidation or thermalexpansion.

The chamber is made in the form of a vertical cylinder to ensurecoverage by the cooling water and to permit effective uniformdissipation of heat. The ratio of height to diameter may be varied tocontrol the temperature of the gases leaving the chamber in thetemperature range in which heat transfer by radiation is effective. Themost effective ratio is from 2:1 to 3:1 which allows maintenance of theexit gas temperature at 1200 to 1800 F.; this makes possible the use ofa shelland tube-type cooler for cooling the gases down to a point justabove the dew point? of phosphorus pentoxide (about 500 F. when burningcondensed elemental phosphorus). The Walls are kept bare and unjacketed,since any jacketing material would reduce the heat transfer rate andcause the inner surface to heat up to a point where the graphite wouldbe oxidized.

The dome-shaped top is covered with steel to avoid putting a head ofwater on vertical or substantially vertical joints between the domeblocks; Because of the relatively large air space between the steel andthe blocks necessary to permit dif-j-i ferential thermal expansion,graphite dome blocks would not be cooled sufficiently even in thisrelatively cool part of the chamber. The refractory in the dome, andalso in the hearth 6 adding Water to the combustion chamber to flush theash into the hydrator.

It should be emphasized that use of graphite, which has never beenregarded hitherto as a suitable material, is made possible only by acertain combination of design features. The chamwhen a refractory hearthis used, is subjectj to much less severe service than in the ordinarycombustion chamber. The large, relatively cool inner surface of thegraphite wall in view of the dome and hearth removes heat from the refractory by true radiation at such a rapid rate that these portions ofthe chamber operate normally at a temperature well below red heat, whichis very favorable to long life. The refractory hearth is subjected to ahigher temperature by deliberate concentration of combustion in thatzone only at infrequent intervals to permit fusion and removal ofaccumulated-ash.

The design of the graphite hearth presented a difficult problem. It isnecessary to cool the hearth and also to provide some safeguardagainstthe possibility of acid leaking out through the joints in the graphitehearth blocks. This was solved by placing the combustion chamber in ametal pan which sits in a pool of water. It was found that. theinterposition of metal between the graphite and cooling water isallowable here, although it is not allowable on the walls or dome,because the Weight of the furnace bottom gives a close enough contactbetween the graphite and metal to give sufficient heat transfer to keepthe hearth cool. It is important that the metal rim of the pan notextend up the wall above the top of the hearth; otherwise a hot spotwill develop on the wall because of the decrease in heat transfer. Because of differential thermal expansion, the metal cannot be made to fitthe wall closely enough to give sufiicient heat transfer.

The graphite-refractory combination has been found to be the onlyfeasible hearth design for use with high-ash phosphorus. If all-graphiteconstruction is used, the occasional concentration of heat in the hearthsection for the purpose of melting out ash deposits would causeoxidation and failure of the graphite. On the other hand, anall-refractory design cannot be used because of the low heat transfercoefficient of the refractory, as discussed above. The reason thatrefractory can be used in the hearth section is that the graphite wallabove it cools the gas to a temperature low enough to preventappreciable damage to the refractory. The hearth is also cooled, asexplained above, by radiation of heat from the hearth to the graphitewall.

The refractory hearth is used only with highash phosphorus; the graphitehearth is preferable for low-ash phosphorus both because it is cheaperto build and maintain and also because it allows cooling the gas to alower temperature. The relatively small amount of ash which 001- lectson the graphite hearth can be removed by ber must be vertical andcylindrical, in the first place, so that a relatively thick, uniformlayer of water can be flowed down the outer surface. Otherwise, the heatwill not be removed fast enough and the graphite will oxidize. Inaddition, cement joints perpendicular to the line of heat flow must notbe used, 1. e., the wall must be only one block thick. This gives anuninterrupted path of graphite for the heat to follow and makes possiblekeeping the inner surface ata low enough temperature to protect thegraphite. Finally, certain features are necessary in the design of thegraphite hearth, as discussed above. By using this combination, a

very unpromising construction material has been used with excellentresults.

The porosity of the graphite presents no problems, which is anunexpected development. The inner wall is cooled to such a degree in acombustion chamber of the above design that solid metaphosphoric acidcondenses on the wall and fills the pores. This prevents appreciableleakage between the inner and outer walls, but it might be expected thata layer of acid would form on the wall surface which would cut down theheat transfer by a significant amount. It has been found, however, thatthis does not occur at the particular Wall temperature used,

The design as shown is intended for use with a countercurrent hydrator,i. e., one in which the cooled combustion gases enter at the bottom andthe water spray comes from the top. If a cocurrent hydrator (one inwhich both gases and water enter at the top) is to be used, the designof the combustion chamber is substantially the same except that thecombustion gases leave the chamber from the top and phosphorus isintrocluced through two or more horizontal atomizin'g burners mounted inthe walls near the hearth.

It will be seen therefore that this invention actually may beconstructed by the use of various modifications and changes withoutdeparting from its spirit and scope.

I claim:

1. In a phosphorus combustion furnace comprising a combustion chamberhaving an outlet for hot combustion gase adjacent to an end thereof, aburner adapted to burn phosphorus disposed in said chamber adjacent toan end thereof farthest from said outlet, and means for introducing airin quantity sufilcient to maintain an oxidizing atmosphere within saidchamber disposed adjacent to said burner, that improvement whichcomprises, in combination, a container for a pool of water; supportingmembers disposed in said container; a flat metal pan disposed in saidcontainer adapted to be supported by said supporting members in intimatecontact with said pool of water in said container; a circular hearthfloor formed of a single thickness of graphite blocks disposed in saidmetal pan; a vertical cylindrical Wall formed from a single thickness ofgraphite blocks disposed upon the periphery of said hearth floor; asubstantially dome-shaped top disposed upon said cylindrical wall to cooerate with said wall and said hearth floor to enclose a verticalcombustion chamber having a ratio of height to diameter of from 2:1 to3:1; a burner adapted to burn phosphorus assess-a 7' centrany disposedin saidcombustien chamber adjacent to said top; an outlet for hot combostiongases disposed adjacent to'sai'd hearth fio'or; and means forcontinuously flowing a film of water of-substantialthickness downwardover the exterior surface of said Wall disposed adjacent to the exteriorof said chamber.

2. #In a phosphorus combustion furnace com-- prising a combustionchamber having -an-o1itlet for hot 'com-bustiongases' adjacent to an-endthereof, a burner adapted to burn phosphorus disposed. in said chamberadjacent to an "end thereof farthest from said outlet, and means-forthan carbon disposed in said metal shell to sur round a lower, minorportion of the combustion chamber; a vertical cylindrical wall formed'flfil?" a single hickness of graphite block's disposed upon theperiphery of said vhearth section; a sub stantially dome-shaped topdisposed upon cylindrical wall to cooperate with said wall and saidlining toenclose a vertical combustion chamher having a ratio of heightto diarneterof from 2:1 to 3:1 and'having a major upper portionbounded-by said cylindrical wall; a burner adapt edt'oburn phosphorusdisposed in the combustion chamber adjacent to said top; an outlet forhot combustion gases disposed in said hearth section;

an Til 6321 8 for conti uously *fiovlfin' a fi'im- Water of substantialthickness over the exterior siirfaceof said wall disposed adjacent tosaid top.

-' CHARLES J. MCFARLIN.

REFERENCES CITED The i'ollo'wing' references are of record in the fileof this patent:

UNITED STATES PATENTS Number N a-me Date 602,74? Harding Apr. 19, 189811709 708 Pistol et a1 Jan. 29, 1929 1,910,101* Flischer ue May 23; 19331,932,954 Conradty' Oct. 31, 1933 2,125,297 Junkins Aug; 2, 19382,132,360 Merchant 0013.4, 1938 2,173,8 9 Curtis et a1 Sept. 26, 19391:2,272,414 McCullough Feb. 10, 1942 FOREIGN PATENTS Number Country Date347,644 Great Britain Oct. 21, 192-9:

OTHER' Ollinger: Recent Developments in Carbon Cheinical Equipment,Chemical Industries, May 1944, pages 683-688;

Carbon and Graphite Products, Catalog Sec. M-BOOG-A, National CarbonCompany, Inc, page l2 and l3. 1

1. IN A PHOSPHORUS COMBUSION FURNACE COMPRISING A COMBUSTION CHAMBERHAVING AN OUTLET FOR HOT COMBUSTION GASES ADJACENT TO AN END THEREOF, ABURNER ADAPTED TO BURN PHOSPHORUS DISPOSED IS SAID CHAMBER ADJACENT TOAN END THEREOF FARTHEST FROM SAID OUTLET, AND MEANS FOR INTRODUCING AIRIN QUANTITY SUFFICIENT TO MAINTAIN AN OXIDIZING ATMOSPHERE WITHIN SAIDCHAMBER DISPOSED ADJACENT TO SAID BURNER, THAT IMPROVEMENT WHICHCOMPRISES, IN COMBINATION, A CONTAINER FOR A POOL OF WATER; SUPPORTINGMEMBERS DISPOSED IN SAID CONTAINER; A FLAT METAL PAN DISPOSED IN SAIDCONTAINER ADAPTED TO BE SUPPORTED BY SAID SUPPORTING MEMBERS IN INTIMATECONTACT WITH SAID POOL OF WATER IN SAID CONTAINER; A CIRCULAR HEARTHFLOOR FORMED OF A SINGLE THICKNESS OF GRAPHITE BLOCKS DISPOSED IN SAIDMETAL PAN; A VERTICAL CYLINDRICAL WALL FORMED FROM A SINGLE THICKNESS OFGRAPHITE BLOCKS DISPOSED UPON THE PERIPHERY OF SAID HEARTH FLOOR; ASUBSTANTIALLY DOME-SHAPED TOP DISPOSED UPON SAID CYLINDRICAL WALL TOCOOPERATE WITH SAID WALL AND SAID HEARTH FLOOR TO ENCLOSE A VERTICALCOMBUSITON CHAMBER HAVING A RATIO OF HEIGHT OT DIAMETER OF FROM 2:1 T3:1; A BURNER ADAPTED TO BURN PHOSPHORUS CENTRALLY DISPOSED IN SAIDCOMBUSTION CHAMBER ADJACENT TO SAID TOP; AND OUTLET FOR HOT COMBUSTIONGASES DISPOSED ADJACENT TO SAID HEARTH FLOOR; AND MEANS FOR CONTINUOUSLYFLOWING A FILM OF WATER OF SUBSTANTIAL THICKNESS DOWNWARD OVER THEEXTERIOR SURFACE OF SAID WALL DISPOSED ADJACENT TO THE EXTERIOR OF SAIDCHAMBER.